In the rubber industry, it has been hitherto known that at the time of producing a rubber composition containing a carbon black, or a carbon black species, a rubber wet masterbatch is used to improve the rubber composition in workability and carbon black dispersibility. This is a technique of: mixing the carbon black beforehand with a dispersing solvent at a predetermined ratio; using mechanical force to mix the resultant carbon-black-containing slurry solution, in which the carbon black is dispersed in the dispersing solvent, with a rubber latex solution in a liquid phase; subsequently adding a solidifier such as an acid to the mixture to yield a solidified product; collecting the product; and then drying the product. In the case of using a rubber wet masterbatch, a rubber composition is obtained which is better in carbon black dispersibility and in rubber properties, such as workability and reinforceability, than in the case of using a rubber dry masterbatch obtained by mixing a carbon black with a rubber in a solid phase. The use of such a rubber composition as a raw material makes it possible to produce a rubber product, such as a pneumatic tire having, for example, a decreased rolling resistance, and an excellent fatigue resistance.
As a method for producing a natural-rubber wet rubber masterbatch, Patent Document 1 listed below describes a natural-rubber wet masterbatch producing method including the step of mixing a slurry solution in which a carbon black is beforehand dispersed in water with a natural rubber latex, the method being a method wherein about the particle size distribution of the carbon black in the slurry solution, the 90% by volume particle diameter (D90) of the carbon black is adjusted to 30 μm or less. As a natural-rubber wet masterbatch rubber composition, Patent Document 2 listed below describes a natural-rubber wet masterbatch rubber composition obtained by a production method of mixing a natural rubber latex with a slurry solution in which a carbon black is beforehand dispersed in water.